Gasification of a solid fuel is one of the most fundamental techniques required for the energy in future, and the fuel and chemical raw material industry. Coal, biomass, and various kinds of solid waste are converted into product gas or synthetic gas by the gasification. The product gas can be used as a fuel of a high-efficiency energy system, or can be converted into synthetic natural gas by further purification and methanation, and can be a high-quality fuel and a chemical raw material instead of natural gas. Further, the synthetic gas is a source gas in various chemosynthesis processes, and can also produce base chemical raw material, such as methanol or dimethyl ether (DME), by synthesis, but also can produce various kinds of fuel oil that substitutes for a petroleum product.
In the situation of the resource environment where petroleum resources are drained day by day, and the price soars, manufacturing the above-described various kinds of industrial sources and fuel from various kinds of solid-state resources (coal, biomass) or raw materials (various kinds of organic matters including waste) using a gasifying means is a flow that is not avoided. Accordingly, the need for a gasifying technique is increasing globally.
Research and development of the gasifying technique already have 100 years or more of history, and various techniques are settled. The features of the main gasifying techniques developed until now and the situation of applications are summarized in Table 1. Although fixed bed gasification has little capitalization and has the feature, such as simple operation, since it is difficult to apply this gasification on a large scale, the gasification is typically applied only to a distributed energy system that uses biomass and waste as fuel. Although the updraft pressurization fixed-bed coal gasification furnace by Lurgi has comparatively large application in chemical fertilizer and synthetic oil production (by parallel installation of several tens of gasifying furnaces), it is considered that most of them transit to other gasifying techniques suitable for the application to enlargement. Since the highest reaction temperature of the fluidized bed gasification (pressurization or normal pressure) is limited to about 900° C., it is impossible to produce only the product gas containing hydrocarbons, such as methane. Accordingly, such gasification is mainly applied to high-efficiency energy production systems, such as an IGCC (pressurization), a gas turbine (pressurization), and a gas engine. As the scale of energy, there are middle-sized applications of several tens of megawatts (MW) through large-large applications of 100 MW or more. Although decoupled fluidized bed gasification is a special fluidized bed gasifying technique and generates reaction heat using air, it has the feature that the generated product gas is not diluted with N2 of combustion air, and CO2 produced by combustion. Accordingly, this technique can be applied to synthetic natural gas (SNG) production of middle energy scale, and large-scale cogeneration systems (for example, the production of fuel gas and tar by pyrolyzing and electric power generation by combustion). The reaction temperature of the entrained flow gasification using pure oxygen is 1300 to 1700° C., and the synthetic gas that does not contain tar but contains hydrocarbons a little is produced. Since temperature is high, pure oxygen is used, pressurization is made, cost is high, and technical difficulty exists, it can be applied only to large-scale systems (>100 MW).
TABLE 1Features of representative gasification techniquesGasificationTechniqueEnergy ScaleApplication TargetImportant ProblemsTwo-step<10 MWHeat supply,Simplification of operation,gasificationDistributedincrease of efficiency,cogenerationremoval of tar (excluding(application todown-draft), increase oflarge-scale iscalorific value, reduction ofdifficult)cost* Fluidized>10 MWGas engine, gasTechnical demonstration,bed (including(typically, 10 toturbine, fuel gas,increase of calorific value,BFB, CFB,100 MW)IGCC (>100 MW)gas refinement in furnace,SB)high-temperature dustremoval, efficiencyoptimization, low calorificvalue combustorDecoupled10 to 100 MW,Gas engine,Technical demonstration,fluidized bedco-productionco-production, fuelefficiency improvement, gas100 MW or lessgas, SNG, H2refinement in furnace,processing of high water-containing fuelEntrained flow>100 MW Chemical synthesis,Development of the present(typicallyH2, IGCCtechnique to be applied tousingnon-coal fuels, such aspressurizationbiomassand oxygen)Note:BFB represents a bubbling fluidized bed, CFB represents a circulating fluidized bed, and SB represents a spouted fluidized bed.
Accordingly, in applications of the gasifying techniques, the decoupled cogeneration based on the fixed-bed gasification in an early period, the concentrated cogeneration and SNG fuel production based on the distributed fluidized bed gasification in a middle period (after five to ten years), the H2 production, chemosynthesis, IGCC, etc. based on pressurized entrained flow/fluidized bed gasification in a long period (after 10 to 15 years), are expected. However, various kinds of gasifying techniques themselves should be further developed. The problems to be solved about each technique are mentioned in the column “Important problems” in Table 1. Asian nations including China and Japan clearly fall behind Western countries in terms of the gasifying techniques, and particularly, the techniques and development ideas with unique features in pressurized gasification, entrained flow gasification, and decoupled fluidized bed gasification are scarce. In view of the fact that the main application of the gasifying techniques exists in China and Southeast Asia, it is considered that it is necessary to make research and development of a new gasifying technique.
The solid fuel gasification represents a reaction complex that combines drying/pyrolyzing of fuel, gasification of char, tar/hydrocarbon reforming, and four combustion chemical reactions that generate the reaction heat required for the chemical reactions. These reactions have their independent functions, are related to one another, and form a complicated chemical reaction network. How this chemical reaction network is managed and utilized is a basic factor that determines the merit of a gasifying technique. Actually, steam and tar are generated by drying/pyrolyzing of fuel. The tar is a fundamental reactant of a reforming reaction and the steam emitted by drying can be used as a reaction agent of two reactions including gasification of char, and tar/hydrocarbon reforming. Simultaneously, when the reforming reaction is performed in the same reaction space as the gasification of char, the char itself can be gasified, and simultaneously can provide a catalytic effect to tar/hydrocarbon reforming reaction. By down-draft fixed-bed gasification, the tar content of the generated product gas can be reduced to several tens of milligrams. This supports the catalyst or promoting effect over tar decomposition of the char. All three reactions including drying and pyrolyzing of fuel, gasification of char, and tar/hydrocarbon reforming are endothermic reactions, and combustion of a combustible gas generated in part of the char or these reactions provides reaction heat exactly required for the reactions. However, when the carbon dioxide (CO2) that is generated by the combustion reaction and air-oxidation agent are used, a larger amount of mixed nitrogen (N2) may become inactive ingredients that dilute the product gas.